This invention relates to powder metal cutting tools, more specifically cutting tools containing tungsten carbide, cubic boron nitride, and a metal binder.
Metal carbide tool bits, such as tungsten carbide with cobalt, reflect a relatively refined art, developed over the past several decades. Typically they are fabricated by mixing, pressing, and sintering constituent powders. Previously many different combinations and variations of compositions have been evaluated and reported in the literature; these are reflective of the search for tougher and longer wearing tool bits.
Cubic boron nitride is a material which has received considerable attention over the past 20 years because in certain applications it outperforms tungsten carbide-cobalt. Cubic boron nitride (CBN) is a material of unique crystallographic structure, approaching diamonds in hardness; it is to be contrasted with the older and more familiar hexagonal boron nitride which is a soft material not suited per se as a cutting material. The properties and manner of preparation of CBN are described in Wentorf, Jr., U.S. Pat. No. 2,947,617. The use of CBN as an abrasive tool is described in De Vries, U.S. Pat. No. 3,918,931; the abrasive body produced is comprised of CBN grains bonded by a transition metal-aluminum alloy or other metal. The process of making CBN and CBN tool bits involves extremely high pressures and is a significant technological step. Accordingly, CBN tool bits are quite expensive compared to the carbide bits which they replace and there is a need for a material which has a better combination of improved performance and cost.
Heretofore nitrides in general, and hexagonal boron nitride in particular, have been incorporated in metal carbide tool bits in an effort to improve performance. For example, the early U.S. Pat. No. of Laise 1,858,247, mentions 0.1-1% boron nitride; Kieffer U.S. Pat. No. 3,741,733 discloses cutting tools comprising nitrides or nitride-carbide mixtures or carbon nitrides with metals of the iron and chromium group. In Kieffer the nitrides are those of the transition metal groups and do not include that of boron which of course is not a transition metal. Wentorf, Jr. et al. U.S. Pat. No. 3,743,489 discloses cutting tools of CBN compacts. The compacts are formed from crystals in the size range of 1-10 micrometers and are bonded together at very high pressures with aluminum containing alloys of various transition element metals. Wentorf, Jr. et al also disclose the utility of CBN compacts, both by themselves and when bonded to conventional sintered carbide cutting tool substrates. Yates U.S. Pat. No. 3,409,416 discloses transition metal nitride base tool bits having metal binders of the refractory metal type and optionally containing substantial amounts of other materials including a carbide such as aluminum and titanium carbide. Boron nitride is mentioned in the Yates patent but it is evident from the disclosure that the nitride is the conventional (hexagonal) boron nitride, with a maximum size of about 30 micrometers.
Based on the above art it can be seen that the cutting tool bits known heretofore which contained boron nitride were comprised of either of two basic constructions: (a) those containing the hexagonal form of the nitride in complex combination with various other materials, wherein because of its lack of inherent hardness the hexagonal boron nitride would not appear to be a significant element in enhanced tool bit wear resistance; and (b) those containing CBN as a compact in combination with a metal alloy, wherein the compact was the tool bit in its entirety or in the alternative was adhered to a hard supporting substructure.
Additionally, it is evident from the art relating to CBN that extremely high pressures, of the order of 5000 MPa, are required to obtain the density and integrity of particle bonding necessary for a good tool bit. Devices to attain such high pressures increase the cost of tool bits and make impossible the fabrication of large ones.